If villagers are descendants of the ancestral deities and the masks hold their power, which God was the Maiden's ancestor? And it was these gods that caused the defilement...and wearing the mask allows one to gain the powers of the deity whose mask it is.
The ceremony happens yearly...and when they wear masks, they live as the Messengers of the ancestral deities of their respective respective villages.
Yoshiro and Soh descended into the human world.
Yoshiro gives Soh a physical form and also becomes the Spirit Stone eventually.
Why is Soh's body related to Yoshiro's well-being? Why doesn't Soh lose their form entirely when her defilement gets worse?
Yoshiro and Soh are names given to the current Maiden and Messenger.
Maiden and Messenger are both roles.
All the masks represent an ancestral deity, including Soh's. Is Soh's mask the Mountain Goddess?
Is Birth of a Goddess showing the Goddess who's the ancestor of the Maiden's family? In the ending, Soh absorbs the Goddess's powers that was trapped within the spirit stones.
But the masks are said to be the power of the Goddess but they're also the masks of the ancestral deities who caused the defilement.
A conclusion on masks...
The masks contain the powers of the Goddess which allows those who wear it to channel the abilities of their ancestors. Ancestors are deified in Shinto hence why they are called 'gods'.
But still no answer as to why Soh still has a form despite Yoshiro's defilement crippling her.